The present invention relates generally to high-speed data communications over conventional telephone lines with or without load coils. More particularly, this invention pertains to a system and method built into a high-speed digital communication transceiver pair for detecting the presence and absence of load coils on a telephone line and adapting each transceiver based on the presence or absence of load coils. The adaptation of each transceiver provides for improved data transmission reliability over both loaded and unloaded lines.
Telephone companies and other providers of communications services have sought in recent years to develop improved hardware and techniques for using existing conventional copper telephone lines, such as twisted pairs, to transmit and receive digital data. One of the services offered by telephone companies is digital data services (DDS) at up to 64 Kbps over four wire unloaded twisted pairs.
One of the disadvantages of transferring data over conventional telephone lines, i.e., over twisted wire pairs, is dealing with a variety of line impairments, including hardware added to the twisted wire pairs that was intended to be used for analog voice services. For example, telephone companies have conventionally attached load coils at periodic intervals along the twisted pairs connecting the central office to the customer premises. The load coils reduce attenuation across the voice frequency band thereby maintaining voice quality over a range of line lengths.
Unfortunately, the load coils also cause a substantial roll-off or attenuation of the frequency response of the twisted pair above 3500 Hz. The roll-off limits the frequency bandwidth available for data transfer using the twisted pair. This can cause significant problems if the twisted pair is used for DDS because the frequency bandwidth necessary for DDS is greater than 3500 Hz.
As a result, service providers must first determine if load coils are present before they can use prior art transceivers with a given twisted pair. If load coils are present they must be removed in order to deploy DDS. Information regarding the presence or absence of load coils on a given twisted pair, however, is not always readily available.
What is needed, then, is a system and method that may be incorporated into a high-speed digital transceiver pair for detecting and compensating for load coils in a twisted pair.
The present invention provides a system and method that is incorporated into a high-speed digital communications transceiver pair and is operable to determine whether or not load coils are connected to a twisted pair. In addition, the system and method automatically adjust the transceiver pair to compensate for the presence or absence of load coils on the twisted pair. The present invention determines whether or not load coils are connected to a twisted pair by transmitting a test signal, which has signal power (i.e., frequency components) concentrated in the voice frequency band and signal power (i.e., frequency components) concentrated outside of the voice frequency band, over the twisted pair. The voice frequency band is defined within the present invention as the band of frequencies ranging from approximately 300 Hz to 3300 Hz. The signal power concentrated in each band is not critical so long as it is possible to distinguish the signal power in one band from the signal power in the other band.
When load coils are connected to the twisted pair, the load coils reduce the attenuation of frequency components inside the voice frequency band, while at the same time, significantly increasing the attenuation of frequency components outside of the voice frequency band as compared to unloaded loops.
By measuring the signal power that passes through the twisted pair it is possible to determine whether or not load coils are connected to the twisted pair. For example, if a signal having frequency components concentrated inside and outside the voice frequency band is transmitted over a twisted pair, and the signal received on the an other side of the twisted pair has frequency components concentrated outside of the voice frequency band that have been significantly attenuated by the transmission through the twisted pair relative to the frequency components concentrated inside the voice frequency band, load coils are connected to the twisted pair. Otherwise, load coils are not present.
In one embodiment, the present invention determines whether or not load coils are connected to a twisted pair by calculating the ratio of the power of the received signal outside of the voice frequency band and the power of the received signal inside the voice frequency band and comparing this ratio to a predetermined constant. If the ratio of powers is greater than the predetermined constant, then load coils are not connected to the twisted pair. If the ratio is less than the predetermined constant, then load coils are connected to the twisted pair.
Once the present invention determines whether or not load coils are connected to the twisted pair, an adjustment circuit is used to adjust the transceiver parameters accordingly. To put it another way, if load coils are detected, the adjustment circuit causes the transceiver to use one set of transceiver circuits and parameters and, if load coils are not detected, the adjustment circuit causes the transceiver to use another set of transceiver circuits and parameters.
In one embodiment, the present invention includes a transmitter connected to a receiver using a twisted pair. The transmitter includes a signal generating circuit that is operable to generate and transmit a test signal that contains a first signal at a first frequency and a second signal at a second frequency. The first signal has a frequency that is higher than the highest frequency in the voice frequency band and the second signal has a frequency that falls within the voice frequency band. In other words, the signal power of the test signal is concentrated inside and outside of the voice frequency band. In one embodiment, the first signal has a frequency of 4 kHz and the second signal has a frequency of 1.5 kHz.
The receiver includes a load coil detection circuit that is operable to receive and filter the test signal, which is modified by transmission across the twisted pair, to separate out the first and second signals. The load coil detection circuit further includes a signal power measurement circuit for measuring the signal power of the first and second signals and a comparator circuit for comparing the measured power of the first signal with a scaled measured power of the second signal. If load coils are present, then the first signal will be more attenuated than it would be if load coils were not present. In a similar manner, the second signal will be less attenuated if load coils are present.
Finally, the receiver includes an adjustment circuit that is used to adjust the receiver based on the presence or absence of load coils on the twisted pair.
In an alternative embodiment, the present invention is incorporated into a pair of transceivers. In this alternative embodiment, the present invention simply uses a transmitter located in the first transceiver and a receiver located in the second transceiver to determine if load coils are present on the twisted pair. Once this determination is made, the adjustment circuit in the second transceiver is used to adjust the receiver in the second transceiver accordingly. In addition, this information is sent to the first transceiver and the first transceiver is adjusted accordingly.
In another alternative embodiment, the present invention uses the transmitter located in the first transceiver and the receiver located in the second transceiver to determine if load coils are present on the twisted pair. In addition, in this embodiment the present invention also uses the transmitter located in the second transceiver and the receiver located in the first transceiver to determine if load coils are connected to the twisted pair. In this case, the adjustment circuits in both the first and second transceivers are used to adjust the transmitters and receivers in each transceiver accordingly.